Primary feed for dish reflector having dielectric lens to reduce side lobes

ABSTRACT

A primary feed for a front-fed aerial system, the feed comprising a mushroom-shaped fitment of dielectric material. The stalk of the mushroom enables the fitment to be coupled to a waveguide feed while the head forms a lens which projects energy towards the aerial system.

United States Patent Inventor Dennis Murdoch Knox High Wycombe, England Appl. No. 809,795 Filed Mar. 24, 1969 Patented Oct. 5, 1971 Assignee Her Majesty's Postmaster General London, England Priority Mar. 25, 1968 Great Britain 14357/68 PRIMARY FEED FOR DISH REFLECTOR HAVING DIELECTRIC LENS TO REDUCE SIDE LOBES 1 Claim, 3 Drawing Figs.

u.s. Cl 343 755,

343/781, 343/783 Int. Cl H0lq 19/10 Field of Search. 343/753,

References Cited UNITED STATES PATENTS Willoughby Bartlett et al. Iams Pippard Plummet et al..

Cutler Sherman Petrich FOREIGN PATENTS Great Britain Primary Examiner-Eli Lieberman An0meyHall & Houghton ABSTRACT: A primary feed for a front-fed aerial system, the feed comprising a mushroom-shaped fitment of dielectric material. The stalk of the mushroom enables the fitment to be coupled to a waveguide feed while the head forms a lens which projects energy towards the aerial system.

PATENTEUHBI awn SHEET 1 [IF 2 DEN/YA) M. lflvax INVENTOR BY fiat/6 4% ATTORNEY PRIMARY FEED FOR DISI-I REFLECTOR HAVING DIELECTRIC EENS TO REDUCE SIDE LOBES B QBQVN 9? THE INYP T QN This invention relates to front-fed aerial systems and has particular although not exclusive reference to front-fed aerials for satellite communication earth stations.

Aerials for satellite communication earth stations are required to have high efiiciency and low noise characteristics. In addition, the radiation patterns of the aerial must have lowlevel side lobes to minimize interference to or from neighboring terrestrial satellite systems. One of the factors atfecting the radiation patterns of the aerial is the radiation pattern of the primary feed to the aerial. Where the primary feed is an openended waveguide, it can be stated that, in general, radiation from the waveguide has a single-lobed pattern with maximum gain along the axis of the feed, the gain reducing towards the periphery of the aerial reflector. Such a single-lobed pattern inevitably produces some spillover at the periphery of the aerial and this gives rise to undesirable side lobes.

It is an object of the present invention to improve the radiation pattern of the primary feed and thereby to reduce the level of side lobes in the radiation pattern of the aerial.

According to the present invention, there is provided for the primary feed of a front-fed aerial system a dielectric lens assembly comprising in combination a coupling portion for coupling the assembly to a waveguide, and a lens portion of dielectric material having a domed face of a thickness measured along the axis of the waveguide of not less than 2dlhe where d is the diameter of the waveguide and he is the wavelength in the dielectric.

The radiation pattern may, for example, have a gain function which is substantially constant within the angular limits of the reflector of the aerial. Alternatively, the pattern may have a gain function which gives maximum efficiency.

The predetermined pattern is such that undesirable side lobes in the radiation pattern of theaerial are minimized.

In one embodiment of the invention the face of the lens portion is coated with a layer of dielectric of thickness equal to one quarter of the wavelength at the frequency for which the lens is designed and of a dielectric constant-V; where e is the dielectric constant of the remainder of the assembly. Such a layer operates to minimize internal reflection at the face of the lens.

Internal reflection at the front face may also be reduced by drilling or otherwise forming a large number of holes in the front face, each hole being a quarter wavelength deep.

The assembly is mushroom-shaped, the stalk constituting the coupling portion and the head of the mushroom being the lens portion. The rear face of the head may be of frustoconical form with an apex angle approximately equal to the angle subtended by the reflector with which the lens is used. This lens assembly may also have the dielectric coating referred to above.

Unwanted radiation caused by internal reflections at the lens face may be reduced by fitting a flange to the exterior of the waveguide adjacent the rear face of the lens.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. I is a section of a first embodiment and part of feed waveguide, and

FIGS. 2 and 3 are similar cross sections of second and third embodiments.

DESCRIPTION OF THE PREFERRED EMBODIMENTS The lens itssemhlysliuwn In FIG. l is of polythene and is of generally mushroom shape, the stalk I constituting a portion for coupling the assembly to a feeder waveguide 2. Portion l is cylindrical because waveguide 2 is a circular waveguide and is tapered to facilitate matching.

The head 3 forms the lens portion of the assembly and its thickness on the axis should be not less than ZJIM (where d is the diameter of the waveguide) so that the front face of the lens lies in the far'field of the waveguide aperture. The contour of the face 4 controls the radiation pattern of the feed and is detennined by ray geometry.

A typical ray emerging from the phase center of the assembly at an angle b, to the axis of the assembly is refracted at face 4 and emerges therefrom at an angle 45. The total power (p) radiated in the solid angle 24 (assuming circular symmetry) is P p= L /(q Sm d where P= total power transmitted from the assembly and G,= the gain function.

The power radiation in a cone of half-angle b, before reshaping is equal to that radiated in a cone of half-angle b, after reshaping.

Therefore, I 0 (4)) sin dd1= I l (I ur sin rIulr/i where G is an average value of gain function.

In practice, it is convenient to evaluate graphically the equation just given. Thus, with a knowledge of the radiation pattern of the waveguide feed without the lens assembly it is possible to create a table of values for I and 0,. The surface .coordinates of the face 4 can then be determined using the normal laws of refraction which may be expressed as M241 and sin 6/sin 7=n= where y and 8 are the angles of incidence and refraction at the face 4 n is the refractive index of polythene and e is the dielectric constant of polythene. The phase characteristic of the primary feed is modified by the presence of the dielectric and is given by radians approximately where r is thelength alori'g any ray from the phase center to the face 4,

r is the length of the on-axis path in the dielectric.

he is wavelength in the dielectric,

A0 is the free space wavelength. 7 v

The edge of the head 3 is chamfered at 5 to minimize spillover, total internal reflection occurring over the chamfer 5. FIG. 1 also shows the dimensions of a lens for use at a frequency of 4 GHz.

Internal reflection at face 4 can be reduced if the face is covered with a layer 6 as shown in FIG. 2. The layer is, in effect, the equivalent of the well-known quarter wave transformer. The inclusion of such a layer will affect the ray geometry to some extent, but this can be allowed for. if desired, when determining the contour of the face 4. V

A reduction in spillover can also be effected by imparting to the rear face 7 of the head 3, the frustoconical configuration shown in FIG. 3. The apex angle of the cone is approximately equal to the angle subtended by the reflector. For use .at a frequency of 4 GI-Iz., the lens shown in FIG. 3 has the dimensions given in FIG. 1 except that the rear face 7 runs back to the stalk l. 1

It will be understood that the face of the embodiment of FIG. 3 can also be coated in the manner described above with reference to FIG. 2.

Unwanted radiation from the lens portion caused by internal reflection at the front face may be reduced by fitting a flange to the exterior of the waveguide adjacent the rear face of the lens. Such a circular disc or flange is shown diagrammuticully only by the dotted rectangle in FIG. I. For use with the lens assembly shown in FIG. I, the disc may have a diameter of about 12 inches.

While it is normally convenient to mould the lens assembly in one piece, this is not essential provided no voids are left. For example, the embodiment of FIG. 3 could be made in two parts as indicated by junction line 8.

path

The lens assembly is not necessarily a solid of revolution as are the embodiments described above. If the aerial reflector is noncircular, different corrections might be required in different planes.

Dielectric lens assemblies embodying the invention can also be used with rectangular waveguides. The design principles of such lens follow the method outlined above ln practice, differing corrections might be required in the E and H planes so that the assembly would not then be a solid of revolution.

1 claim:

1. A dielectric lens assembly for the primary feed of a frontted aerial system, the assembly comprising in combination a waveguide coupling portion consisting of a cylindrical part which fits closely in the waveguide anda tapered part, a lens portion attached to the coupling portion, the lens portion having a substantially domed face and being of a thickness. measured in a direction along the axis of said cylindrical part. not less than 2a"/)te where d is the diameter of the waveguide and he is the wavelength in the dielectric, the thickness of the lens 

1. A dielectric lens assembly for the primary feed of a frontfed aerial system, the assembly comprising in combination a waveguide coupling portion consisting of a cylindrical part which fits closely in the waveguide and a tapered part, a lens portion attached to the coupling portion, the lens portion having a substantially domed face and being of a thickness, measured in a direction along the axis of said cylindrical part, not less than 2d2/ lambda e where d is the diameter of the waveguide and lambda e is the wavelength in the dielectric, the thickness of the lens portion in a direction parallel to said axis being less over the center of the face and at the edges thereof than elsewhere, and a rear face chambered at an angle to minimize spillover with respect to the reflector of the aerial system with which the lens assembly is to be used. 